At least three major, noninterconvertible, tetrameric isozymes of pyruvate kinase (EC 2.7.1.40) occur in mammals. Isozymic hybrids can be produced in vitro or occur in vivo when two subunit types are synthesized simultaneously in the same cell. We are examining the cellular distributions of the isozymes by immunofluorescence and cell separations and their molecular properties by kinetic, chemical, and physical characterization. We are also utilizing their striking kinetic differences to study allostery and subunit interactions. Studies of pyruvate kinase isozymes are of direct medical importance in several major areas: (a) genetic aberrations of erythrocyte pyruvate kinase, which may be related to the liver isozyme or may represent a fourth form, result in a type of nonspherocytic anemia; (b) inhibition of brain pyruvate kinase by high levels of phenylalanine has been proposed as an important factor in the brain damage resulting in untreated cases of phenylketonuria; and (c) shifts in pyruvate kinase isozymes occur during differentiation and dedifferentiation and therefore might be useful for detecting certain pathological conditions such as liver damage or tumor growth. Through characterization of pyruvate kinase isozymes and their hybrids, we hope to better define their roles in normal carbohydrate metabolism and in clinically important abnormal conditions. BIBLIOGRAPHIC REFERENCES: Strandholm, J.J., Dyson, R.D. and Cardenas, J.M. (1976) Bovine pyruvate kinase isozymes and hybrid isozymes. Electrophoretic studies and tissue distribution. Arch. Biochem. Biophys., 173, 125. Dyson, R.D., Cardenas, J.M., Tolle, S.W., Newburgh, R.W., Richards, T.C., and Garnett, M.E. (1976) Pyruvate kinase isozymes during developmental and malignant changes in neurons, glia, and hepatocytes. Federation Proceedings 35, 1403 Abs.